Hydroxyl Radical Generation Depending on O2or H2O by a Photocatalyzed Reaction in an Aqueous Suspension of Titanium Dioxide

Photocatalytic production of the electron (e(-)) and positive hole (h(+)) in an aqueous suspension of TiO2 (anatase form) under illumination by near-UV light (295-390 nm) generated the superoxide (O2 (-)) and hydroxyl radical (•OH), which both proceeded linearly with reaction time, while H2O2 accumulated non-linearly. Under anaerobic conditions (introduced Ar gas), the yields of three active species of oxygen were decreased to 10-20% of those detected in the air-saturated reaction. The electron spin resonance (ESR) signal characteristics of •OH were obtained when a spin trap of 5,5-dimthyl-1-pyrroline-N-oxide (DMPO) was included in the illuminating mixture. The intensity of the ESR signal was increased by Cu/Zn superoxide dismutase, and decreased under anaerobic conditions, amounting to only 20% of the intensity detected in the aerobic reaction. The addition of H2O2 to the reaction mixture resulted in about an 8-fold increase of •OH production in the anaerobic reaction, but only about 1.5-fold in the aerobic reaction, indicating that e(-) generated by the photocatalytic reaction reduced H2O2 to produce •OH plus OH(-). On the other hand, D2O lowered the yield of •OH generation to 18% under air and 40% under Ar conditions, indicating the oxidation of H2O by h(+). The addition of Fe(III)-EDTA as an electron acceptor effectively increased •OH generation, 2.3-fold in the aerobic reaction and 8.4-fold in the anaerobic reaction, the yield in the latter exceeding that in the air-saturated reaction.